The article, "Enhanced Differential GPS Carrier-Smoothed Code processing Using Dual Frequency Measurements," proceedings of ION GPS-98, The 11.sup.th Int. Tech. Meeting of the Satellite Div. of the Inst. of Navigation, Nashville, Tenn., Sept. 15-18, 1998, by p. Y. Hwang, G. A. McGraw and J. R. Bader, is herein incorporated by reference.
Global navigational satellite systems (GNSS) are known and include the global positioning system (GPS) and the Russian global orbiting navigational satellite system (GLONASS). GNSS-based navigational systems are used for navigation and positioning applications In the GPS navigational system, GPS receivers receive satellite positioning signals from a set of up to 32 satellites deployed in 12-hour orbits about earth and dispersed in six orbital planes at an altitude of 10,900 nautical miles. Each GPS satellite continuously transmits two spread spectrum, L-band signals: an L1 signal having a frequency f1 of 1575.42 MHz, and an L2 signal having a frequency f2 of 1227.6 MHz. The L1 signal from each satellite is modulated by two pseudo-random codes, the coarse acquisition (C/A) code and the p-code. The p-code is normally encrypted, with the encrypted version of the p-code referred to as the Y-code. The L2 signal from each satellite is modulated by the Y-code. The C/A code is available for non-military uses, while the p-code (Y-code) is reserved for military uses.
GPS navigational systems determine positions by timing how long it takes the coded radio GPS signal to reach the receiver from a particular satellite (e.g., the travel time). The receiver generates a set of codes identical to those codes (e.g., the Y-code or the C/A-code) transmitted by the satellites. To calculate the travel time, the receiver determines how far it has to shift its own codes to match the codes transmitted by the satellites. The determined travel times for each satellite are multiplied by the speed of light to determine the distances from the satellites to the receiver. By receiving GPS signals from four or more satellites, a receiver unit can accurately determine its position in three dimensions (e.g., longitude, latitude, and altitude). A conventional GPS receiver typically utilizes the fourth satellite to accommodate a timing offset between the clocks in the receiver and the clocks in the satellites.
In conventional GPS receivers, code and carrier pseudorange information has been combined in a complementary filter to suppress multipath and code tracking noise. However, the amount of smoothing, specifically the length of the smoothing time constant, is limited by the dynamics of ionospheric refraction. The smoothing process works best when the ionospheric component is constant. However, this is seldom the case for low-elevation satellites or when there is considerable ionospheric disturbance in the upper atmosphere. This problematic effect is widely known as ionospheric divergence, and arises because the standard form of carrier smoothing with single-frequency measurements is not sufficient to eliminate the ionospheric component.
The limitation of conventional carrier smoothing processing techniques, which is ionospheric divergence between code and carrier, gives rise to a residual ranging error that is proportional to the smoothing filter time constant. Thus, there is a trade-off between the iono-induced smoothing error and the amount of attenuation that can be achieved of slowly-varying code errors such as multipath.